Intrinsic vacancy induced nanoscale wire structure in heteroepitaxial Ga$_2$Se$_3$/Si(001)*

A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga$_2$Se$_3$) on the lattice matched substrate, arsenic-terminated Si(001). This anisotropic, nanowire structure is attributed to surface coalescence of the intrinsic vacancies in $\beta$-Ga$_2$Se$_3$, a defected zinc-blende semiconductor with every third Ga site vacant. Scanning tunneling microscopy of Ga$_2$Se$_3$ films reveals nanoscale, wire-like structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect bulk $\beta$-Ga$_2$Se$_3$, which contains ordered $\langle 110\rangle $ arrays of Ga vacancies. The semiconducting, passivated wires form perpendicular to the underlying As-dimer rows of the Si(001):As substrate, and continue to lie in the same direction, with bilayer height differences, as the film grows thicker.